Are you sure? I always play with 640 x 480 (yes... performance over quality. I'm paranoid with resolution, so screw blurry texts) on high-demanding games, Renegade X included. As I said before, I do believe I only spot SBHs when they're 5 feet away. So unless I'm near-sighted (which I am, my right eye is near-sighted, but I wear glasses) I don't think it has anything to do with resolution
Honestly, I have no clue what is causing it, I'm just guessing here.